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i
3D electron microscopy investigations of human dentin and ion beam
irradiation effect on biocompatible anatase TiO2 using Focused Ion Beam
based techniques
by
Sina Sadighikia
Submitted to the Graduate School of Engineering and Natural Sciences
in partial fulfillment of
the requirements for the degree of
Master of Science
Sabancı University
December 2015
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iii
©2015 by Sina Sadighikia ALL RIGHTS RESERVED
iv
3D electron microscopy investigations of human dentin and ion beam irradiation effect
on biocompatible anatase TiO2 using Focused Ion Beam based techniques
Sina Sadighikia
Materials Science and Nano-Engineering, MSc Thesis, 2015
Supervisor: Prof. Mehmet Ali Gülgün
Co-Advisor: Dr. Meltem Sezen
Keywords: FIB/SEM, 3D Electron Microscopy, Electron Tomography, Ion Beam
Irradiation, Human Dentin, anatase TiO2
Abstract
The essence of this study, in addition to the three dimensional image reconstruction of
human dentin microstructure in micro and nano size; involves irradiation effects and
modification of anatase TiO2 surface by gallium focused ion beam, the various properties
of modified surface were investigated by means of Scanning Electron Microscopy (SEM),
Raman spectroscopy and Energy Dispersive Spectroscopy (EDS). The aforementioned
procedures can successfully be carried out using a dual-beam system consisting of high-
resolution scanning electron microscope (HR-SEM), focused ion beam (FIB) columns,
attachments such as gas injection systems (GIS), and detectors for elemental analysis
(EDS). However, the ion beam irradiation causes some artifacts along with other
beneficial modifications on material’s surface especially on biocompatible materials such
as TiO2. Therefore, in this study we considered the limitations as well as the advantages
of using focused ion beam for nanostructuring, ion implantation etc.
v
In the first part of this study, high-resolution electron microscopy techniques, such as
Focused Ion Beam (FIB), Scanning Electron Microscopy (SEM) and High Resolution
Transmission Electron Microscopy (HRTEM) revealed micro and nano features within
human dentin with high definition and accuracy. The samples were prepared using FIB
based advanced nanostructuring processes in a dual-beam instrument. The related
secondary electron (SE) image tomographs were acquired by means of stacking the
images from FIB slice-series for monitoring micro-sized dentinal tubules, whereas FIB-
structured pin-like samples were investigated at the TEM to observe the collagen fibrils
at the nanoscale. The complementary analysis helped to reveal the microstructure and
morphology of human dentin in three dimensions in detail.
In the second part of the study, surface morphology and microstructural evolution upon
low energy ion irradiation of anatase TiO2 were investigated by in situ focused Ga+ ion
beam/scanning electron microscopy. A surface roughening through pore formation,
coalescence and eventually nanoneedle formation were induced on TiO2 surface. The
mechanism of nanoneedle formation was investigated. In addition, Raman spectroscopy
and EDS analysis of irradiated surface revealed the gallium implantation during direct
milling. Gas assisted etching was investigated in this study in order to reveal the
enhancement of surface milling in presence of assisting gas.
vi
3D electron microscopy investigations of human dentin and ion beam irradiation effect
on biocompatible anatase TiO2 using Focused Ion Beam based techniques
Sina Sadighikia
Malzeme bilimi ve nano-mühendisliği, MSc Tezi, 2015
Danışman: Prof. Mehmet Ali Gülgün
Ortak Tez Danışmanı: Dr. Meltem Sezen
Anahtar kelimeler: FIB/SEM, üç boyutlu Elrktron Mikroskopi, Elektron Tomografi,
İyon Demeti Işınlama, Insan Dentini, Anataz TiO2
Özet
Bu çalışmanın kavramı, dentinin üç boyutlu yeniden yapılandırma mikro ve nano boyutta,
hem de ayrıca galyum odaklı iyon ışını tarafından ışınlama etkilerini anataz TiO2
yüzeyinde ve modifikasyonu içerir. Bu aşamada Taramalı Elektron Mikroskobu (SEM),
Raman spektroskopisi ve Enerji Dağılım Spektroskopisi (EDS) vasıtasıyla modifiye
yüzeyin farklı yönlerini araştırıldı. Yukarıda bahsedilen girişimlerin bir çift ışınlı sistemi
kullanılarak gerçekleştirilebilir. Şu cihaz yüksek çözünürlüklü taramalı elektron
mikroskobu (HR-SEM), odaklanmış iyon demeti (FIB), element analizi detektörü (EDS)
ve gaz enjeksiyon sistemiyle (GIS) oluşmuş. Ancak, iyon demeti ışınlama faydalı
değişiklikler sürece bazı eserlere malzemenin yüzeyinde neden oluyor Özellikle biyo-
uyumlu TiO2 olarak malzemeler üzerinde. Bu nedenle, bu çalışmada, avantajlar yanında
bu tür sınırlamalar göz önünde alındı.
Bu çalışmanın ilk bölümünde, yüksek çözünürlüklü elektron mikroskopi teknikleri
(Odaklı İyon Işın (FIB), Taramalı Elektron Mikroskobu (SEM) ve Yüksek Çözünürlüklü
Transmisyon Elektron Mikroskobu (HRTEM)), yüksek çözünürlüklü ve doğruluk ile
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insan dentinin içindeki mikro ve nano özellikleri ortaya çıkardı. Numuneler, bir çift ışınlı
cihazda FIB tabanlı gelişmiş nanostructuring işlemleri kullanılarak hazırlanmıştır. Ilgili
ikincil elektron (SE) görüntü tomografları FIB dilim serisi görüntüleri istifleme suretiyle
mikro-boyutlu dentin tübüllerini izlenmesi için elde edildi. Halbuki FIB yapılandırılmış
pin-benzeri örnekler nano kollajen fibriller gözlemlemek için TEM de incelenmiştir.
Tamamlayıcı analiz mikroyapı ve detaylı olarak üç boyutlu insan dentinin morfolojisini
ortaya çıkarmak için yardımcı olur.
Çalışmanın ikinci bölümünde, yüzey morfolojisi ve mikrostructure evrimi düşük enerji
iyon ışınlama anataz TiO2 üzerinede FIB ile incelenmiştir. Gözenek oluşumu yoluyla
pürüzlendirme bir yüzey birleşme ve sonunda nanoneedle oluşumu, TiO2 yüzeyi üzerinde
uyarılmıştır. Nanoneedle oluşum mekanizması incelenmiştir. Ek olarak, Doğrudan
öğütme sırasında galyum implantasyonu, Oksijen boşluk oluşumunu ve amorfizasion
Raman ve EDS analizleriyle ortaya çıkdı. Gaz destekli gravür Bu çalışmada araştırıldı.
viii
Acknowledgment
Firstly, I would like to express my sincere gratitude to all who have never hesitated to
support me by any means possible to fulfill this goal.
I would like to gracefully thank my advisor Prof. Mehmet Ali Gülgün.
Besides my advisor, I would like to express my special appreciation and thanks to my Co-
advisor Dr. Meltem Sezen.
I would also like to thank my committee members, professor Melih Papila, professor
Sedat Alkoy, for serving as my committee members.
I also would like to express my deep gratitude to all my friends who supported me in
writing, and incented me to strive towards my goal.
Finally yet importantly, I wanted to salute the support of my lovely family who has always
supported me by any means necessary.
Sina Sadighikia
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To all genius people who found some meaning in the suffering
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Table of Contents
Abstract ............................................................................................................................ iv
Özet .................................................................................................................................. vi
Acknowledgment ........................................................................................................... viii
Table of Contents .............................................................................................................. x
List of Figures ................................................................................................................. xii
List of Tables .................................................................................................................. xv
Motivation ...................................................................................................................... xvi
1. Introduction ............................................................................................................... 1
Scanning Electron Microscopy (SEM) .............................................................. 1
Focused Ion Beam (FIB) Microscopy ................................................................ 8
Dual-Beam (SEM/FIB) Systems ...................................................................... 13
1.3.1.1. TEM Specimen Preparation .............................................................. 15
1.3.1.2. Slice & View ..................................................................................... 17
1.3.1.3. 3D Microstructural Characterization and FIB-Tomography ............. 17
Electron and ion beam irradiation on various materials................................... 20
1.4.1.1. Electrostatic charging ........................................................................ 21
1.4.1.2. Atomic displacement (Knock-on) ..................................................... 21
1.4.1.3. Electron beam sputtering ................................................................... 22
1.4.1.4. Electron beam heating ....................................................................... 22
1.4.1.5. Radiolysis (Ionization damage) ......................................................... 23
1.4.1.6. Hydrocarbon contamination .............................................................. 23
1.4.2.1. Sputtering .......................................................................................... 26
1.4.2.2. Amorphization ................................................................................... 26
1.4.2.3. Gallium implantation ......................................................................... 28
1.4.2.4. Specimen Heating .............................................................................. 29
1.4.2.5. Redeposition ...................................................................................... 29
xi
1.4.2.6. Swelling ............................................................................................. 30
2. Material and Methods ............................................................................................. 32
Dentin and Enamel ........................................................................................... 32
Titanium dioxide (TiO2) .................................................................................. 34
2.2.1.1. Structural properties .......................................................................... 35
2.2.1.2. Raman vibration properties ............................................................... 37
Characterization and Analysis.......................................................................... 38
3. Experimental Results and Discussions ................................................................... 50
3D Reconstruction of human dentin via slice and view technique .................. 50
Electron Tomography of human dentin and enamel ........................................ 53
Ion Beam irradiation effects analysis on nanophase TiO2 anatase in FIB ....... 57
Surface morphology evolution during ion beam irradiation ............................ 58
Amorphization, gallium implantation and oxygen vacancy production during
ion beam irradiation .................................................................................................... 62
Gas assisted etching of anatase surface ............................................................ 68
4. Conclusion .............................................................................................................. 71
5. References ............................................................................................................... 72
List of publications ......................................................................................................... 79
Refereed Journal Publication ...................................................................................... 79
Refereed Conference Papers and Abstracts ................................................................ 80
xii
List of Figures
Figure 1 Basic SEM column ............................................................................................. 3
Figure 2 Signal generation due to specimen-electron beam interaction ........................... 5
Figure 3 Edge effect in imaging with secondary electrons ............................................... 5
Figure 4 The interaction volume of electron beam and the specimen .............................. 7
Figure 5 A typical Everhart-Thornley detector ................................................................. 8
Figure 6 An illustrative representation of a LMIS .......................................................... 10
Figure 7 a) LMIS socket with filament; b) apex region of the filament with needle .... 11
Figure 8 Cross section of a basic FIB column ................................................................ 12
Figure 9 A DB system configuration .............................................................................. 14
Figure 10 The dual-beam FIB system (Jeol JIB 4601F) ................................................. 15
Figure 11 TEM specimen preparation of TiO2 rutile using a dual beam tool, coarse
milling, lift out, mounting and thinning steps ................................................................. 16
Figure 12 a) Schematic depict of a conventional sample for 3D image reconstruction b)
SE image of a desirable sample for serial sectioning (taken from[19]) ........................ 18
Figure 13 3D EDS image reconstruction which shows particle distribution in three
dimensional volume (taken from[20]).......................................................................... 19
Figure 14 Examples of in-situ mechanical testing by FIB: a) comparision tests on a gold
pillar (taken from[24]) b) yield strength/ plasticity test on nickel superalloy pillar (taken
from[22]) ........................................................................................................................ 19
Figure 15 Irradiation damage classified according to electron scattering behavior (taken
from[25]) ........................................................................................................................ 20
Figure 16 Schematic illustriation of collsion cascade generated in a crystal lattice by a
Ga+ incident ion (taken from[31]) ................................................................................ 25
Figure 17 Schematic of etching material’s surface with ion beam ................................. 26
Figure 18 TEM images of the amorphized surface on different surfaces (taken from[33])
........................................................................................................................................ 27
Figure 19 The comparison of formed amorphous layer in different ion energies. (taken
from[34]) ........................................................................................................................ 27
Figure 20 TEM image showing the implanted Ga+ into the Silicon surface (taken
from[38]) ........................................................................................................................ 28
Figure 21 AFM measurement showing the swelling and material removal processes with
respect to ion dose (taken from[38]) ............................................................................. 31
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Figure 22 The scheme of a human tooth showing the individual layers ........................ 33
Figure 23 SEM micrograph showing dentinal tubules and porous structure of dentin
(taken from[47]) ............................................................................................................ 33
Figure 24 SEM image of enamel surface morphology after laser and phosphoric‑acid
treatment (taken from[49]) ............................................................................................ 34
Figure 25 TiO2 crystal structure comparing two distinct phase of rutile and anatase (taken
from[60]) ........................................................................................................................ 36
Figure 26 TiO2 crystal structure with detailed information (taken from[58]) .............. 37
Figure 27 Raman spectra of nanoparticle TiO2. a) rutile b) anatase .............................. 38
Figure 28 Example Raman spectra of various molecules (taken from[63]) ................. 40
Figure 29 Comparison of Raman scattering interpretation (taken from[63]) ............... 41
Figure 30 Schematic showing the model of diatomic molecule as a mass on a spring
(taken form[63]) ............................................................................................................ 41
Figure 31 Jablonski diagram representing the transitions for various scattering (taken
form[63]) ........................................................................................................................ 44
Figure 32 Comparison of raman spectrum at various excitation wavelengths (taken
from[63]) ........................................................................................................................ 47
Figure 33 Typical Design of a Raman Probe (taken from[63]) .................................... 48
Figure 34 SE image showing the human tooth and the dentin layer around the channels.
The selected areawith the yellow square shows the region where the FIB sectioning was
done. The FIB tomography carried out on the region has dimensions of x = 18 µm, y =
17 µm and z = 16 µm ...................................................................................................... 51
Figure 35 The SE images showing the distribution and alignment of the tubules for 3 axes:
red arrow correspond to x axis, whereas blue arrow represent the y-axis and green arrow
the z axis. The image on the left hand side shows the cross-sections of the tubules along
the y axis and the image on the right hand side shows the tubules along the x axis. ..... 52
Figure 36 The 3-D reconstructed of dentin showing the tubule distribution in 3
dimensions: red arrow correspond to x axis, whereas blue arrow represent the y-axis and
green arrow the z axis. For the reconstruction Stack N-Viz software was used. ............ 52
Figure 37 SE images showing the FIB cross-section of the enamel layer: the cross-cut on
the surface and magnified SE image for observing the prisms....................................... 53
Figure 38 The steps for preparation of pin-like TEM sample using the dual-beam
instruments: (a) deposition of electron beam assisted Pt layer, (b) deposition of ion beam
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assisted Pt layer, (c) ion beam milling via annular patterns, (d) lift-out of the pre-section,
mounting of the pre-section onto the grid; (f) final thinning and polishing. .................. 55
Figure 39 The Bright Field (BF) TEM images showing the 3D distribution of collagen
fibrils within the human dentin. The micrographs show the nano features within the dentin
structure. ......................................................................................................................... 56
Figure 40 The Bright Field (BF) TEM images showing the crystalline structure of human
dentin. ............................................................................................................................. 56
Figure 41 morphology and topography of as-compacted sample ................................... 59
Figure 42 The sequence of morphology evolution and nanoneedle formation during ion
beam irradiation a) increasing the raoughness of surface b) .......................................... 60
Figure 43 Showing the nanoneedle formed under ion bombardment. The diameter of the
nanoneedle at the tip is around 50 nm ............................................................................ 60
Figure 44 Comparison of formed nanoneedles under two different ion milling angle a)
normal to the surface b) 15º tilted c) 35º tilted d) 55º tilted ........................................... 61
Figure 45 Raman spectra of as-compacted and irradiated samples ................................ 64
Figure 46 Red shift in Eg(3) characteristic peak due to ion implantation ...................... 64
Figure 47 EDS analysis of as-compacted and irradiated anatase ................................... 65
Figure 48 TRIM simulation of Ga+ ion penetration into TiO2 surface in zero tilt ......... 66
Figure 49 TRIM simulation of Ga+ ion penetration into TiO2 surface in 55º tilt ........... 66
Figure 50 SEM image analysis showing the material removal versus ion dose and
different phenomena occured during ion beam irradiation of anatase surface ............... 67
Figure 51 Peak broadening of main characteristic peak of anatase due to oxygen vacany
........................................................................................................................................ 68
Figure 52 Comparison of surface roughness and amorphization between a) direct milling
b) gas assisted etching..................................................................................................... 69
Figure 53 Raman spectra of as-compacted and gas assisted etched anatase surface ...... 69
Figure 54 EDS mapping of irradiated TiO2 surface in presence of XeF2 ....................... 70
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List of Tables
Table 1 srtuctural parameters of TiO2 (taken from[61]) ............................................... 36
Table 2 Ion doses calculated upon beam exposure times and currents .......................... 58
xvi
Motivation
The use of Titanium Dioxide in biocompatible materials is becoming increasingly
attractive for biological applications. Among various phases of TiO2, Anatase and Rutile
are the most extensively studied, due to their stability and vast variety of applications in
energy storage and electrical materials. In addition, anatase is one the most significant
materials in photocatalytic materials. In case of biotechnology, human dentin is one the
most interesting materials in human body, which has gained a great, interest in materials
characterization studies. Using TiO2 based alloys in human tooth, as a biocompatible
implant needs an understanding of human tooth and especially human dentin structure.
On the other hand, electron microscopy can be considered as a comprehensive and
feasible technique concerning the microstructural and chemical analysis, as well as the
modification of such biomaterials. Transmission electron microscopy enables a detailed
investigation of the structures of these materials on a nanometer scale. Dual beam
instruments, consisting of a scanning electron microscope (SEM) and a focused ion beam
(FIB) column, additionally equipped with gas injection system and micromanipulators
serve as multi- functional tools both for device modification and specimen preparation
for TEM.
The irradiation of TiO2 with ions can lead to temporary or permanent changes of its
structure. The modification of the surface morphology of materials with ion beam
irradiation has gained interest in materials science field. Obtaining a surface morphology
with high specific surface area is always desired in various materials science fields such
as biotechnology and energy storage materials.
The first part of the experimental study includes the three dimensional imaging and
investigation of human dentin in order to reveal the interior microstructure of this
material. For this reason the 3D image reconstruction technique of Focused Ion Beam
(FIB) was used to explore the micron sized features inside the human dentin and
furthermore the electron tomography studies has been carried on these features known as
tubles in order to have a three dimensional structure view in nanoscale.
Another part of the study was focused on the analysis of the ion beam irradiation of TiO2
during etching of the surface of this material by FIB. Raman spectroscopy technique was
used in order to reveal the ion beam irradiation effects on TiO2 surface including
xvii
amorphization, oxygen vacancy production and gallium implantation. A study of surface
morphology evolution during ion beam irradiation has been carried out on TiO2 surface
in anatase phase. An interesting nanostructure known as nano-needle was obtained during
ion beam irradiation. This nano-needle structure could play a significant role in
biocompatible materials such as implantation for human tooth due to its high specific
surface area.
1
1. Introduction
Scanning Electron Microscopy (SEM)
The scanning electron microscope (SEM) and its various detectors are one of the most
multifaceted instruments with different applications from the investigation of the
morphology of surface microstructure to chemical composition characterizations[1].
Among different types of electron microscopes that produce images due to electron
sample interaction gained information, the SEM is an instrument for imaging the sample
surface by scanning it with high-energy electron beam. The advantage of SEM over
transmission electron microscopy (TEM) is the ability of SEM to image and analyze bulk
specimens[2]. Since the electrons hitting the sample obtain high range of energies, in the
case of thin specimen, they may have enough energy to transmit through it unabsorbed.
These electrons can produce valuable information about the specimen, which is used to
produce images in TEM. In case of a thicker specimen, electrons are no longer able to
transit through the specimen and the information confines to different particles (e.g.
electrons, x-rays and photones) rising from the surface. These signals are used in a typical
SEM. The information which can be collected from an SEM can be categorized in the
range of surface topography, crystalline structure, chemical composition and electrical
behavior of the top 1 µm of the specimen.
SEM electron optics
A typical SEM column is shown in Figure 1. Within this column, due to the voltage
difference between cathode and anode (ranging between 0.1-50 keV) electrons from a
thermionic, Schottky or field emission cathode are accelerated.
By demagnifying the smallest virtual cross-section of the electron beam near the cathode
with electron optics, a SEM produces a small electron probe at the specimen. In SEM
system parameters such as electron prob size, aperture and the current are directly
dependent on the gun brightness. To date several types of electron guns have been
developed:
Tungsten hairpin filaments are the most common ones. It acts by
producing thermal emission of electrons from its tip. For this aim, the
filament is thermally excited (via applying high electrical currents) to
around 2500 ̊C.
2
The lanthanum hexaboride (LaB6) filament is also a thermal filament and
works by thermionic emission. However, its work function is lower than
for a tungsten filament, so it is more efficient. Advantages comprise of a
bigger maximum beam current, a brighter beam and a longer life time.
However they are more expensive.
Field emission guns (cold cathode emitters) due to the very high electric
field and quantum mechanical tunneling of electrons on finely pointed tip
a beam with high brightness with very small deviation in electron energy
can be obtained. While thermionic guns require a vacuum of about 10-6
Torr, FEGs require lower than 10-10 Torr pressures to protect the tip which
adds extra costs (for vacuum systems) for using these types of guns.
Schottky emitters are known for their high brightness (108 A/cm2) and high
current stability. A Schottky emission cathode consists of a ZrO coated
tungsten wire with a tip radius of 0.1-1 µm. The work function is dropped
from 4.5 eV to 2.7 eV by applying a ZrO coating. The low work function
enables the electrons to emit at a temperature of 1800 K.
3
For demagnifying the electron beam into a fine probe in an SEM column, 2 to 3
electromagnetic condenser lenses are embedded. Via scan coils the electron beam is
scanned across a particular zone of the sample surface. An electron lens comprises of an
axial magnetic field with rotational symmetry. However, there are some limitations
related to lens aberrations, which affect the quality of electron-probe and have to be
considered. There are three types of aberration, which are important in scanning electron
microscopy:
Spherical aberration, which occurs when electrons which are parallel to
the optic axes but at different distances from the optic axes fail to converge
to the same point.
Chromatic aberration is a consequence of a lens having a different
refractive index for different wavelengths of lighting. This results in
focusing of different wavelengths at different points.
Figure 1 Basic SEM column
4
Astigmatism is another important type of lens defect, which happens due
to magnetic inhomogenities of the pole pieces. If an optical system whit
astigmatism is used to form an image of a sphere, the vertical and
horizontal lines will be in sharp focus at two different distances, which
may end up with an elliptical shape.
The electron spot size determines the spatial resolution of the SEM. This electron spot
size depends on several parameters. Among these parameters, the wavelength of the
electrons and the electron-optical system, which produces the scanning beam, play a
significant role. Another important parameter, which restricts the resolution, is the
dimensions of the interaction volume [3].
Signals and Imaging in SEM
The interaction of electron beam whit a bulk sample causes repeated scattering and
absorption of electrons due to lose of energy. These phenomena occur within a teardrop-
shaped volume of the sample (100 nm to 5 µm) up to the surface and known as the
interaction volume. The electron beam energy is the determining parameter of the
interaction size; however, other parameters such as the atomic number of the specimen
and the specimen’s density are also affecting it. The main information about the surface
topography comes from the inelastically scattered electrons, which are produced due to
energy exchange between the electron beam and the sample. These electrons are known
as secondary electrons. The reason for emission of electromagnetic radiation and
elastically scattered electrons is the energy exchange between the electron beam and the
sample. In order to image the sample various signals from the specimen can be gathered
(Fig. 2) as follows:
Secondary Electron (SE) images: The electrons, which escape from the
specimen with lower energies (usually lower than 50 eV), are known as
secondary electrons. These electrons chiefly thumped out of their orbits
around an atom (usually from the k-shell) by an incident electron. Since
these electrons escape from an exceptionally shallow, close surface layer
of sample, provide the highest spatial resolution images, which contain
information about the morphology and topography of material’s surface.
However, since a few back-scattered electrons also collected by secondary
5
electron detector, some compositional contrast is also present. The
contrast is dominated by the so-called edge effect: more secondary
electron can leave the sample at edges leading to increased brightness
there (Fig. 3)
Figure 2 Signal generation due to specimen-electron beam interaction
Figure 3 Edge effect in imaging with secondary electrons
6
Backscattered Electron (BSE) images: Those electrons, which have
enough energy to approach the nucleus of an atom adequately nearly
usually, scatter through a large angle and energies. These electrons are
known as backscattered electrons (BSE). Since they come from a region
a little bit deeper than where secondary electrons come, the images
produced by backscatter electrons have slightly less resolution. For the
most part, they give compositional data: elements of higher atomic mass
give brighter contrast. Backscattered electrons can likewise give
crystallographic data, as electron channeling occurs.
Electron Beam Induced Current (EBIC): in semiconducting specimens,
incident electrons may generate several electron-hole pairs. Ordinarily,
most recombine within about 10-12 seconds. Be that as it may, if an electric
field splits the electrons and holes before they can recombine, an impelled
current flow between the electrodes will occur, leading to the formation
of an EBIC image.
Cathodoluminescence (CL): Light emitting is one of the probable
consequences when electron-hole pairs produced by the incident electron
beam recombine. The wavelength relies on upon the band gap energy of
the sample and in this manner on the composition. Prior to measuring by
an appropriate identifier, the signal may pass a spectrometer. This strategy
is magnificent for uncovering defects that debase radiative characteristics.
Catodoluminescence signals originate from the entire specimen-beam
interaction volume, so have a resolution of about 1 nm.
Voltage-contrast imaging: There is difference between secondary image
of a semiconductor produced with applied voltage and one with no
voltage; the potential created over the dynamic regions changes the
quantity of secondary (low energy) electrons released from those areas.
Extra electrons can eject from active zones where a negative voltage is
produced, so these appear brighter regions where a positive voltage is
created.
7
Auger electrons and x-rays: After an internal shell excitation, an atom
possesses an energy higher than its relaxed state. There are two main ways
out of several ways that the atom can relax and release some of this energy.
Both begin with an outer electron jumping in to fill the vacancy in the
inner shell. Characteristic X-ray emission: Energy is radiated as a single
X-ray photon. Auger electron emission: Energy is given off by one of the
outer electrons leaving. It conveys a characteristic kinetic energy. Auger
electrons are emitted from atomic layers very close to the surface and give
significant information about the surface chemistry. Measurement of the
energies (or wavelengths) of these x-rays gives information about the
chemical composition of the specimen. Characteristic x-rays are emitted
from the entire specimen-beam interaction volume (Fig. 4). There are two
different ways to detect the x-rays: energy dispersive or a wavelength-
dispersive spectrometer. One of the most common attachment to SEMs
for qualitative analysis of the specimen is Energy dispersive x-ray
spectroscopy (EDS or EDX). This information can be used for 3D
quantitative analysis of specimens as well.
The Everhart-Thornley detector is the most effective detector for secondary electrons
(SE) (Fig. 5). Electrons are collected by a positively biased grid in front of a scintillator
biased at +10 kV. The light emission is recorded by a photomultiplier tube.
If the solid angle of collection is increased, the scintillation detectors can also be used for
backscattered electrons (BSE). Other choices for BSE are semiconductor detectors,
microchannel plates or the switch of BSE to SE.
Figure 4 The interaction volume of electron beam and the specimen
8
Figure 5 A typical Everhart-Thornley detector
For producing images, there are some electronic devices, which are used to reveal and
magnify the signals. For displaying them as an image, it uses a cathode ray tube in which
the raster scanning is synchronized with that of the microscope. The image is a result of
a distribution map of the intensity of the signal being emitted, which is acquired from the
scanned area of the specimen. The image can be captured either by photography from a
high-resolution cathode ray tube or digitally on a computer monitor.
In an SEM, the ratio of the dimensions of the raster on the specimen and the raster on the
display device defines the magnification. Assuming that the display screen has a fixed
size, higher magnification results from reducing the size of the raster on the specimen,
and vice versa. Magnification is therefore controlled by the current supplied to the x-y
scanning coils, and not by objective lens power[2].
Focused Ion Beam (FIB) Microscopy
Focused Ion Beams (FIB) is one of the outstanding technologies as far as site-specific
analysis, imaging, milling, deposition, micromachining, and transmission electron
microscopy (TEM) sample preparation of materials are concerned. For more
convenience, FIB instrument may be incorporated into other analytical instruments. The
most versatile of those is a dual beam platform of FIB /SEM will be introduced in the
next section.
9
There are some minor differences between FIB instrument and scanning electron
microscope (SEM) and generally, they are similar to each other, except that the beam is
rastered over the sample is an ion beam rather than an electron beam[4]. The main
components of a simple single FIB instrument are vacuum system and a chamber, a liquid
metal ion source, an ion column, a sample stage, detectors, gas supply and transfer system
and a computer for running the whole apparatus.
As in most cases, implementation of ion beam for investigations necessitates the use of
vacuum systems. For the simplest FIB instrument, having two vacuum pumping regions,
one for the source and ion column and one for the sample and detectors are necessary.
Nowadays, for more convenience a third system is also utilized for faster sample
exchange. Like field emission SEM sources a FIB require a vacuum i.e., approximately
1x108 torr to prevent contamination of the source and to avoid electrical discharges in the
ion column while applying high voltages. Lower vacuum levels (i.e. 1x106 torr range) are
acceptable for the sample chamber however; higher pressures (1x104 torr range) will lead
to the interaction of the ion beam with gas molecules. This is mainly due to the reduction
of the mean free path at high pressures where the ions can no longer bisect the distance
to the sample without undergoing collisions with the gas atoms or molecules. Considering
these facts, due to their ability to provide higher vacuum levels, ion pumps are suitable
choices for the ion source compartment while turbomolecular pumps backed with suitable
rotary pumps are the typical choices for the sample and the exchange compartments [5].
In common FIBs, a liquid metal ion source (LMIS) is used for generation of the ion beam
with a diameter of about 5 nm. Generally, a typical LMIS is a tungsten (W) needle which
is connected to a metal source reservoir. Numerous pure and alloyed metallic sources are
available for being used in LMIS. But due to some important advantageous which has
been described in the few upcoming lines, gallium (Ga) has been preferred to other metal
sources in commercial FIB instruments:
i. The low melting point of Gallium (Tm = 29.8 C) which minimize any
counteraction or inter-diffusion of the liquid metal to the tungsten needle.
ii. Low evaporation rate of the Ga at its melting point, which yields to a durable
source (can be used for longer time).
10
iii. Viscous behavior on the (usually W) substrate is boosted by the low surface
free energy.
iv. Gallium has exceptional mechanical, electrical and vacuum properties.
v. High angular intensity with a small energy spread due to the emission
characteristics of gallium.
In gallium LMIS, gallium metal located in such a way, that it has contact with a tungsten
needle and heated. The emission of the Ga+ occurs in two stages: Primarily, gallium wets
the tungsten needle with a tip radius of about 2-5 µm, and an extensive electric field
(greater than 108 volts per centimeter) causes ionization and field emission of the gallium
atoms in the shape of a Taylor cone. Due to the electrostatic and surface tension force
balance that is set by applying electric field, the aforementioned conical shape is formed.
Then, once this force balance is achieved, because of the very fine tip of the cone, the
extraction voltage pulls gallium from the tungsten tip, resulting in the ionization by field
evaporation of the metal at the tip of the Taylor cone [6].
The current density of the ejected ions is around 108 A/cm2.For creating the Taylor cone,
which results in emission current, a finite voltage is needed. The source is generally
operated at low emission currents (about 1-3 µA) to reduce the energy spread of the beam
and to yield a stable beam. The intrinsic properties of the material and its quantity in the
melt pool play a significant role in the lifetime of a LMIS, which can usually be stated in
terms of µA-hours per mg of the melt (molten metal). An average lifetime for a source
gallium is around 400 µA-hours/mg.
Figure 6 An illustrative representation of a LMIS
11
Once the Ga+ ions are extracted from the LMIS, they are accelerated down to the ion
column to an energy ranging from 5-50 keV and then focused onto the sample by
electrostatic lenses. Figure 8 shows a schematic of the FIB column. The ion column
normally consists of two lenses, a condenser lens and an objective lens. The earlier is the
probe-forming lens and the later is employed for concentrating the beam of ions at the
surface of the specimen. A wide range of beam currents is attainable (few pico ampers to
20-30 nA). Adjusting the beam shape can be accomplished by centering each aperture,
tuning the column lenses, and fine-tuning the beam via utilization of the stigmators.
Cylindrical octopole lenses are exploited to perform multiple functions such as beam
deflection, alignment and stigmation adjustment. Additionaly, the scan field may be
rotated by means of octopole lenses.
The produced ion beam by an ion source is not really pure. Although the source produces
the desired ion species, still due to contamination of the fuel there are other ions present,
in addition there are some contributions of material by other parts in the source and fuel
components. Moreover, the ions are subject to deflection by a magnetic field or an electric
field, which are moving in a stream. For any particular magnetic or electric field, different
ion species are directed along known but different paths. Furthermore, the selected ion
species can be directed along a preselected path, even a straight line, when the correct
orientation and field strength of both the electric and magnetic fields are employed. In
such an ion analyzer, the electric and magnetic fields are at an angle to each other, usually
at right angles to the ion path. Due to this orientation, they are commonly called E cross
B filters, which is in jargon written as ExB[7].
The focusing of the beam is carried out via magnetic lenses in both SEM and TEM. The
Lorenz force is much lower for the ions because they are much heavier and slower. Hence,
magnetic lenses become less effective for the ions in comparison with electrons of the
same energy. Therefor magnetic lenses have been replaced by electrostatic lenses in FIB
a b
Figure 7 a) LMIS socket with filament; b) apex region of the filament with
needle
12
systems. Another reason for this is the fact that Ga+ ions have two isotopes and in case of
using electromagnetic lenses there will be two different confocal points.
Figure 8 Cross section of a basic FIB column
Field variation is negligible for the samples with varied topography because FIB systems
benefit from large working distance (~ 2 cm or less). The provocation of different species
such as sputtered atoms and molecules, secondary electrons and secondary ions happens
when gallium cations hit the sample. More information on this subject will be given in
the subsequent parts of this thesis, where specimen-beam interaction is discussed.
Even a precisely confined ion beam suffers from larger energy spread in comparison with
an electron beam (around 5 eV). Due to the massiveness of the ions compare to electrons,
space charge effects restrain the seeming source size and widens the energy distribution
of the producing ions. This means, the dominant restricting parameter in the resolution of
a FIB instrument is its chromatic aberration. Other important aberrations in a lens system
may be spherical aberration and astigmatism, which were already mentioned in the
previous section for SEM.
In order to have an image in FIB two different kind of detectors which collect secondary
electrons can be used, a multi-channel plate (MCP) or an electron multiplier (EM). A
13
MCP is typically attached straight overhead of the specimen. On the other hand, the EM
is regularly aligned to one side of the ion column. Both secondary electrons and secondary
positive ions which are ejected from the sample are detectable by means of the electron
multiplier detector. It is noteworthy that even during imaging with FIB the sample is being
sputtered; thus tiny beam currents (
14
Figure 9 A DB system configuration
To enable the use of electron beam and ion beam on the matching area, dual beam
instruments usually have a coincident focus point, eucentric point, where both beams
cross from an identical point. Reaching this point requires setting the working distance to
the eucentric height, which again varies for different platforms and can be a distance from
5 mm to 9 mm for the electron beam at 52 ̊ to 55 ̊.
In addition to electron and ion columns, a commercial FIB may be equipped with other
systems as shown in Figure 10, which provide additional in-situ processes within the
microscope such as spectroscopy, deposition, etching and manipulation.
In order to have the capability of site-specific deposition of metals or insulators Gas
injection systems (GIS) can be used in conjunction with the beams. This capability can
enhance the etching process. Metals such as, platinum or tungsten may be deposited by
electron or ion beam assisted CVD of a precursor gas containing these metals in their
structures such as C7H17Pt or C9H16Pt. Principally in a beam-induced deposition the
precursor gas covers the surface of the target substrate and incidence of the beam leads to
its decomposition, releasing the metal on the surface[8]. Finally, dissociated molecules
are adsorbed and deposited to the defined patterns in dual-beam systems.
Dual-beam systems can also be used in acquiring chemical spectra and elemental maps
by means of energy-dispersive spectroscopy (EDS). By combining controlled ion milling
with chemical mapping, three-dimensional chemical reconstructions can be obtained[9].
15
Figure 10 The dual-beam FIB system (Jeol JIB 4601F)
Using an integrated dual-beam system, many feasible issues can successfully be carried
out concerning material science, nanotechnology, semiconductor technology and
biosciences. Some of the frequently used dual-beam techniques will be presented in the
following section.
Dual Beam Applications
1.3.1.1. TEM Specimen Preparation
Sample preparation for transmission electron microscope (TEM) investigation is one of
the most significant applications of a dual-beam tool. For a reasonable penetration of a
beam of electrons, TEM samples must be uniformly thin. with FIB we can produce,
uniformly thick, site-specific samples. In case of composite materials comprised various
organic and inorganic substances FIB provides a possibility to fabricate lamella.
The major benefits of utilizing an FIB for TEM sample preparation are:
It is possible to select the target very precisely with FIB. It has the capability
to prepare lamella with a spatial accuracy of within about 20 nm.
Compare to other techniques TEM sample preparation with FIB is fast and
reliable; it can be vary in the range of 20 minutes to a maximum of 2-4
hours, specimens can be prepared with almost no restriction in terms of
material variety.
TEM sample preparation with FIB based techniques is not dependent on the
target substance. Due to optimization of the geometry and properties of the
16
protective layer on the sample surface it is virtually possible to perform the
milling process on various materials [10].
The most straightforward method for TEM specimen preparation which is shown in
Figure 11, is also called ʻʻin-situ lift-out technique’’[11]. The procedure is based on
several steps starting with a deposition layer of a protecting gas such as platinum or
carbon conducted either by ion or electron beam. Milling two opposing trenches with the
Ga+ ion source and leaving a 1-2 µm thin section is the next step. The procedure goes on
with cutting the bottom and the side trenches away until the section is hold by the bulk
sample from its shoulders. Then, the section can be welded to the micromanipulator by
means of ion beam assisted platinum deposition (IBAD). The next step involves cutting
away the shoulders, now the lamella is free and can be lifted out from the sample,
transferred and attached to a TEM grid. Afterwards, final thinning and polishing in a
thickness range of
17
Although it is possible to prepare TEM samples from various kind of materials by FIB
but this method is often applied for hard materials (e.g. metals, glass, ceramics) or layered
structures (e.g. semiconductors) with hard substrates (silicon, glass, etc.). Soft materials
are sensitive to ion milling due to beam damage and heat dependent shape distortion. For
soft materials, ultramicrotomy[15] is considered to be the most convenient preparation
technique, which is a mechanical sectioning process using a diamond knife.
1.3.1.2. Slice & View Method for 3D Imaging
The possibility of using the ion and electron beams simultaneously in most of the dual
beam instruments opens a way to perform cross-sectioning by means of ion milling and
to acquire electron beam images of the cross-section from the same region of the
specimen. This method is called ʻʻslice and view’’ and often used for the investigation of
multilayered or semiconductor samples. Compared to TEM specimen preparation this
method is quite fast and convenient.
Collecting the data for three dimensional image reconstruction of volumes utilizing a dual
platform FIB/SEM tool as a stack of 2D scanning electron microscopy (SEM) images can
be performed in two different ways: in static or dynamic mode. In dynamic mode hence
the name, SEM images are acquired in real time during the FIB milling process. In static
image acquisition mode, after the FIB milling the ion beam either paused or stopped then
the SEM image can be required which has high resolution due to slow scan[16].
With the application of slice and view process, it is possible to utilize dual beam systems
for failure analysis in semiconductor devices. For instance, Volinsky et al. showed in their
paper the identification several failure mechanisms in memory arrays, including milling
patterns[17].
1.3.1.3. 3D Microstructural Characterization and FIB-Tomography
Dual beam instrument has the ability to provide three-dimensional information
methodologies in order to have a reliable quantitative materials characterization.
Specifically, with 3D imaging and characterization it is possible to measure a number of
crucial geometric properties that cannot be attained utilizing a 2D analysis, such as the
number of particles per unit volume, pore connectivity, real particle shapes and sizes and
spatial dispersion information[18].
18
The basic principle for 3D tomography is collecting continuous 2D data from the surface
of the bulk material by serial sectioning and combining them into a 3D volume by means
of several computational processes (Fig. 12). Serial sectioning is applied by creating a
planar surface by means of removal of the material volume by ion milling. For 3D image
reconstruction, an area of interest is chosen and ion milling is used to create a trench
around this area with high beam currents (5-20nA). The trenches must be adequately large
in order to avoid redeposition of sputtered material and eliminate shadowing of imaging
signals. A protective platinum film (about 1 µm thick) is deposited on the top surface of
the volume of interest before trenching to avoid Ga+ implantation. After these steps,
serial-sectioning process can be initialized.
For complete removal of the total volume, the current and the milling time should be
selected in a way to fulfill this fact. With the removal of each section, various signals
could be collected depended on desired data. These data consist of SE images, BSE
images, EBSD maps and/or EDS data which can be acquired from the specimen surface.
All these data can be used for 3D reconstruction and 3D material characterization.
Consequently, dual beam microscopes are capable of high-fidelity characterization of the
morphology, crystallography and chemistry of micron- and submicron- sized features in
3D[18].
Figure 12 a) Schematic depict of a conventional sample for 3D image reconstruction b)
SE image of a desirable sample for serial sectioning (taken from[19])
19
Figure 13 3D EDS image reconstruction which shows particle distribution in three
dimensional volume (taken from[20])
There are even more applications for FIB/SEM dual beam platform such as
micromachining, nanostructuring and mechanical testing. For instance recently some
works have done for testing the influence of sample dimensions on mechanical properties
known as “size-scale effects”, particularly on metals and alloys[21], [22]. Besides the
preparation of the test structure, FIB allows to conduct in-situ mechanical testing in
micro/nano size when the system is equipped with mechanical-test-stage and nano-
indentation devices. These mechanical properties include tensile strength[23], [24] and
yield strength measurements[22] which is giving out the data in form of stress-strain
curves.
a b
Figure 14 Examples of in-situ mechanical testing by FIB: a) comparision tests on a gold
pillar (taken from[24]) b) yield strength/ plasticity test on nickel superalloy pillar (taken
from[22])
20
Electron and ion beam irradiation on various materials
Electron beam irradiation
Affecting the organic or inorganic samples by electron beam when they are placed in
electron microscope is unavoidable. The main effects can be electrostatic charging,
ionization damage (radiolysis), displacement damage, sputtering, heating and
hydrocarbon contamination. Two major parameters are important in electron beam
damage: first, the amount of radiation damage, which is proportional to the electron dose
and second the extent of damage, which is dependent on the amount of energy, deposited
in the specimen[25].
Figure 2.1 shows the classification of electron beam induced sample damage according
to scattering behavior. All of these effects will be discussed briefly in the subsequent
paragraphs.
Figure 15 Irradiation damage classified according to electron scattering behavior (taken
from[25])
The effect of the electron beam produced by both TEM and SEM may cause different
temporary or permanent changes within the specimen. The scattering behavior of the
21
electrons plays a crucial role in the sort of damage. There are two distinct way of electron
scattering when the electron beam hits the sample: elastic and inelastic scattering. Elastic
scattering (also called as Rutherford scattering) represents the electrons, which are
deflected in high angle by atomic nuclei of host atoms. In this case, the energy of deflected
electrons is conserved and gives rise to electron diffraction patterns and backscattered
electron images. However, it can also result in atomic displacement and sputtering of
atoms within the structure. In the other hand, the inelastic scattered electrons are the
electrons, which have interaction with the electrons of a host atom. In this case, the
deflection angle is low and the initial energy of the incoming electrons has been changed.
This kind of scattering results in production of secondary electrons, x-ray emission and
electron energy loss spectra (EELS). The disadvantage is that all the inelastic processes
the deposition of energy during inelastic interaction is almost unavoidable which can
damage beam-sensitive specimens and leads to radiolysis, which causes structural
changes and material loss[25].
1.4.1.1. Electrostatic charging
The accumulation of surface charge on or inside the specimen results in a phenomenon
known as charging. Basically when the energy from the primary electrons is retained by
the sample instead of being shed to an electrical ground, charging occurs. To avoid this
issue, normally the poorly conducting specimens are coated by a conductive layer, which
eliminates the image artifacts, which appear due to the exuberance surface charge during
SEM analysis. The artifacts shows themselves as irregular, featureless bright patches, or
streaks on SEM images and are generally follow by loss in resolution[26].
Electrostatic charging of low electrical conductive samples incorporates both elastic and
inelastic scattering since the net charge added to the film per second depends both on the
backscattering coefficient and on secondary electron yield[25].
Also in this study, it was necessary to coat the human dentin and Anatase (usually by
sputtering) with conductive layers in order to avoid charging effects during SEM
examinations. Even a few nanometers of metallic coatings were sufficient to see
remarkable differences in SEM image quality.
1.4.1.2. Atomic displacement (Knock-on)
22
Although in elastic scattering the initial energy of the electrons is conserved, but there is
still some amount of energy transferred to the nuclei of host atoms due to momentum
transfer. The knock-on damage occurs when this energy exceeds the displacement energy.
In this case the incident electron generates vacancies e.g. Frenkel defects due to knocking
out an atom from its position by striking it. The energy required for the knock-on process
varies with the atomic number of the sample. This is not a significant issue in SEM
because the energy threshold for knocking out an atom is considerably high. For example,
for carbon with atomic number of 6 the knock-on threshold energy is about 80 keV, and
this energy for silicon (Z=14) the knock-on threshold is 220 keV[25].
1.4.1.3. Electron beam sputtering
When the atoms are knocked out of the specimen to the vacuum due to the interaction
whit electron beam, we call it electron beam sputtering which is usually a high-angle
elastic scattering process. This transferred energy in the electron beam sputtering is very
similar to atomic displacement despite the fact that the transferred energy is much lower.
Here the incident energy needed for sputtering should pass the sublimation energy of the
atoms. According to the study from Egerton and Malac[25], the threshold energy for
electron beam sputtering increases with the atomic number (Z), and therefore sputtering
is most likely to happen for the elements with low atomic numbers.
To avoid this problem limiting the irradiation dose can be helpful. In another case where
there is a need to use high doses, coating the sample with a heavy element as a protective
layer can solve the problem.
1.4.1.4. Electron beam heating
As we mentioned above inelastic scattering takes place when the electron beam collide
with the electrons of host atoms. The transferred energy may convert to heat during this
process and cause a local temperature rise. However, thermal damage is not usually a
serious problem for the materials with medium density and the thermal diffusivity as the
energy deposited is quite small, and the ultimate temperature rise with energy and beam
dose is minimal.
Due to incorporating of various variables, measuring specimen heating experimentally is
difficult. The experimental variables that can affect the result include energy and current,
thermal conductivity, surface condition, thickness and the beam size.
23
1.4.1.5. Radiolysis (Ionization damage)
Another effect of inelastic scattered electrons is the breakage of the molecules bonding
due to ionization. This dissociation of molecules by radiation is known as radiolysis.
In radiolysis process, molecule changes in form and shifts in position and the chemical
bonds are broken so the material loss the crystallinity and as a result swelling and
shrinkage are the major problems in ionization damage, which may cause even material
loss.
There are different ways to overcome this problem. The most cheap and convenient way
is coating the specimen with a metal layer. Besides, since the radiolysis is a temperature
dependent process, cooling the specimen down to cryogenic temperatures can be a
solution during electron microscopy applications.
1.4.1.6. Hydrocarbon contamination
Incoming electrons may cause the polymerization of hydrocarbon molecules on the
surface of a sample. In this case, mass gain occurs on the sample. The low vapor pressure
and low surface mobility are the characteristics of this polymer layer which cause increase
in thickness as the irradiation goes on[25]. It is nearly inevitable to have hydrocarbon
molecules on the surface of specimen and these are typically formed in the vacuum of the
microscope as a pressure of hydrocarbons or silicon oils from the diffusion pump[2].
Although the vacuum systems are alleged clean instruments, but they always have
specific amount of hydrocarbon debris that the vacuum pumps do not adequately remove.
The pump type plays a crucial role in cleanliness of the vacuum and the amount and nature
of these debris molecules.
The diffusion of most hydrocarbons along the specimen can be the source of most
contaminations. The shape of this diffusion is in a way that the hydrocarbons diffuse along
the specimen’s surface towards the edge of the irradiated area and immobilized.
There are many techniques for overcoming this problem and removing the
contaminations. Heating the specimen with an electric lamp, which desorbs hydrocarbons
from the surface, is one of these techniques. Exposing the specimen to the ions in plasma
cleaner or inside the microscope is another solution. In addition using a cold finger inside
the specimen chamber to reduce the mobility of hydrocarbons can be helpful. It is
noteworthy that anything that reduces charging also reduces contamination, therefore
24
coating or pre-cleaning the samples prior to observation would help in minimizing
hydrocarbon contamination[25].
Ion beam irradiation
There are various ways of interaction between an energetic ion, which enters a target, and
the material. Based on the energy of ions, this interaction can be sputtering,
amorphization, swelling, deposition, redeposition, implantation, backscattering or nuclear
reaction. These interactions which are not divisible may point to undesirable side effect
that need to be perceived and prevented for a particular application[27]. The high
radiation damage is also induced since all of these events occur simultaneously. In this
case, not only the morphology changes, but also we have changes in intrinsic physical
properties (conductivity, electrostatic change, elasticity and crystallinity) and chemical
characteristics (composition and hydrophilicity) of the surface[28].
Due to massiveness of ions compare to electrons, these particles cannot readily penetrate
inside individual atoms of the sample and they can gain a high momentum. A 30 keV Ga+
ion depending on the material can penetrate to a depth of 5-40 nm when it hits a surface.
Since these ions have interaction with the atoms of materials they loss their energy and
momentum. This causes the atoms to vibrate profoundly inside their lattice or even to
break, which results in a collision cascade. It is possible to have many independent binary
collisions within the material due to the collision cascade model. A critical amount of
energy called displacement energy is needed to be transferred from the ions to the target
atom in order to knock the atom out of its position. In this process, interstitial-vacancy
pair in a crystalline sample will be introduced. Respectively the energy of this displaced
atom may be sufficient to force out further sample atoms, this phenomenon generates a
volume where a large number of atoms have exuberance kinetic energy. It is noteworthy
that the displacement energy is much larger than the binding energy of the atoms (20 keV
with respect to 1 keV for binding energy)[29].
Local temperature increase may cause amorphization and recrystallization in bulk
materials however the melting temperature of material is related to the sputter yield. The
sputter yield is a measure of the efficiency of the material removal, which defined as the
number of atoms ejected per incident ion[27]. This factor is respectively high for a
material with low melting temperature[30]. In addition to material removal, the ion
25
impact can cause a damaged layer on the specimen surface, which may extent several tens
of nanometers into the material[9].
The transferred energy from the ions to the material may have divers impacts on sample
such as specimen damage, specimen heating, electromagnetic radiation, electron
emission, atomic sputtering and ion emission, ion reflection and backscattering as an
outcome.
Figure 16 Schematic illustriation of collsion cascade generated in a crystal lattice by a
Ga+ incident ion (taken from[31])
Implantation of the ion occurs when the incoming ion rests in the solid. All of these
processes are important to dual-beam system application. Elastic and inelastic interactions
are both incorporate in transferring the ion kinetic energy and momentum the specimen.
Elastic interaction also called electronic energy loss is a kind of interaction in which the
ions lost their energies to the electrons of host material, which results in ionization, and
the emission of the electrons and electromagnetic radiation from the specimen. On
contrary, in inelastic interaction the transferred energy is in the form of translational
26
energy, which screens the target atoms and can result in damage (displacement of atoms
from their initial stage) and sputtering from the specimen surface[31].
In the subsequent sections, main mechanism for ion irradiation will be discussed.
1.4.2.1. Sputtering
The primary mechanism for material removal is known as sputtering. The efficiency for
this material removal process is typically represented by sputter yield (Y), which can be
described as the number of ejected atoms per incident ion. There are different parameters,
which affect the sputter yield; generally, it raises with the ion energy. However, the
incident angle plays a significant role in sputter yield. It is well known that the sputter
yield increases by increasing the incident angle up to 80 ̊ where the yield is maximized,
and then it diclined very briskly to zero as the incident angle approaches to 90 ̊. Besides
it is also dependent on target material. In general, the materials, which obtain low surface
binding energies, can produce higher sputter yield as well as heavier ion sources.
Figure 17 Schematic of etching material’s surface with ion beam
1.4.2.2. Amorphization
Ion bombardment of a desired region on the specimen is the FIB procedure to selectively
remove the material. In this way ion implantation can produce an amorphous phase
surface. Amorphization occurs when the incident ion beam energy or dose is not enough
to cause sputtering and remove the material. In this situation, the bombarded crystalline
substrate may swell[27]. The amorphous phase formed in crystalline materials by ion
27
bombardment is normally meta-stable, and its production depends on unit cell size,
intricacy of chemical ordering and the width of an intermetallic phase field[32].
The amorphization is a serious problem when FIB is used to prepare TEM samples. The
amorphization of crystalline materials may lead to misinterpretation of the structures in
TEM investigations. Because of this, there are extensive studies for overcoming this
problem and minimizing the amorphous layer formation during FIB milling.
Figure 18 TEM images of the amorphized surface on different surfaces (taken from[33])
It is feasible to minimize this damage with using low energies of ions due to
corresponding interaction volume. Cooper et al have shown that the amorphous layer
formation reduces by decreasing the ion energy (Fig. 19)[34].
Figure 19 The comparison of formed amorphous layer in different ion energies. (taken
from[34])
When low energies like 30 keV is used in FIB which is conventional Ga doping stays
almost entirely inside the amorphous layer; however at even lower energies it is possible
28
that the gallium doped region extends beyond the amorphous depth. Therefore, it is
noteworthy that polishing or low energy milling of TEM lamella will still cause Ga
doping which can be a very rich layer as contamination even though imperceptible
amorphous silicon remains on the surface[35].
1.4.2.3. Gallium implantation
The local composition may be influenced by gallium implantation since it can be mixed
into the specimen because of the sputtering process. Kiener et al have found that the
concentration of up to 20% gallium can be found several nanometers below the surface
whereas gallium contents of more than 2% were observed inside a depth of up to 50 nm.
They have measured these concentration depth profiles of implanted gallium by Auger
electron spectroscopy[21].
Gallium implantation has various effects on the sample. It may affect thermal, electrical,
optical, and mechanical properties as well as causing structural changes. Datesman et al
revealed that as because of gallium implantation there is a decrease in transition
temperature and partial increase in resistance of a 10 nm niobium film[36]. In addition it
was shown by Kiener et al[21] that the mechanical properties could be affected by the
gallium occupancy. Although gallium implantation is a destructive phenomenon but
sometimes it can be intentionally used for structural and compositional modifications. For
instance, gallium can be used as dopant implanted into a silicon substrate to locally
modify the conductivity of silicon[37]. Also in this work a physical dope of gallium was
induced on the surface of TiO2 (Anatase) to modify the phase and band gap of the anatase.
Figure 20 TEM image showing the implanted Ga+ into the Silicon surface (taken
from[38])
To prevent this problem in TEM specimen preparation usually a protective layer of
platinum or other protective gas is deposited on the desired region however ion beam
induced deposition (IBID) may implant gallium on the substrate. This may affect the
29
TEM interpretations due to irradiation dependent structural changes. In order to overcome
this issue a protective layer of platinum is deposited on the substrate by means of electron
beam known as electron beam induced deposition (EBID) prior to an ion beam induced
deposition[39]. This EBID layer can be accepted as a non-destructive deposition method
due to the weightlessness of electrons compared with ions.
1.4.2.4. Specimen Heating
Apart from a fraction of energy needed for defect generation and emission of energetic
particles while ion implantation nearly all of the kinetic energy is ultimately transformed
to heat. The ion beam can be as a constant heat source for times more than approximately
a nanosecond and lengths longer than around 100 nm. Due to the shortness of time (less
than 10-12 s), during heating which cause large temporal variations the atoms the
interaction of atoms with each other is scarce and the temperature of the solid is not well
determined. There are different parameters which determine the maximum temperature
that a sample can reach. These parameters are: beam power P, sample thermal
conductivity ҡ sample geometry and contact to a heat reservoir. In below equation a
represents the radius of the circular ion profile on the surface.
𝑇 = 𝑃/(𝜋𝑎ҡ )
For materials with good thermal conductivity, this temperature rise is entirely negligible
but for samples with poor thermal conductivity, this can be an enormous value. For
instance, the temperature increase for silicon with thermal conductivity of 148 W/mK is
< 2 ̊C even for very high doses. On the other hand, for polymers and biological materials
with thermal conductivity of generally 0.1 W/mK this value is much higher[4].
1.4.2.5. Redeposition
From the perspective of thermodynamics, the sputtered atoms and ions, which are thrown
out from the solid surface into the gas phase, are not in equilibrium. In case of any
collision with nearby solid surfaces, the sputtered particles can be condensed back into
the solid phase and a fraction of ejected atoms may crash back into the sputtered surface
and redeposit on it[27] .
Different aspects of sputtered particles such as their charge, mass, kinetic energy and
sputtering direction should be understood to prevent and control redeposition. It is also
30
noteworthy that the sputtering yield of target material and the geometry of milled pattern
are key parameters in intensity of redeposition.
Still there are several methods to overcome this issue. Some of the most important ways
are: using a protective layer, lowering the incidence angle of the encroaching Ga+ ions,
lowering the ion energies, and optimizing milling geometries. Another but not a common
way to reduce the redeposition effect is using high atomic mass ions as ion beam source.
As an example using In+ instead of Ga+[40] .
In TEM specimen preparation by FIB to prevent redeposition usually a low energy of ion
beam polishing is conducted as the last stage with energies about 5 keV and currents
around 70 pA.
1.4.2.6. Swelling
During FIB patterning, amorphization and ion implantation are two main factors of ion-
induced swelling. However, the capability of these mechanisms is different based on the
material. For instance swelling which occurs on crystalline semiconductors is the direct
result of material amorphization, in this case ion implantation does not seem to markedly
commit to volume expansion. The applied pressure from the crystal on the amorphized
material cause the volume expansion towards the surface or swelling. At higher
irradiation doses, the prominent process is surface erosion instead of swelling[41].
Frey et al[38] have shown that, although at low ion doses no material removal will be
observed, but the implanted gallium would lead to swelling of the exposed area. Then,
with the increasing dose, material removal will start. The material removal shows a dose-
dependent behavior as plotted in figure 2.6 (c).
31
Figure 21 AFM measurement showing the swelling and material removal processes
with respect to ion dose (taken from[38])
The participation of amorphization is more than ion implantation factor since the volume
change caused by swelling is far way more than the volume of the implanted atoms. The
swelling due to amorphization can be tens of nanometers and is an important deliberation
in nanofabrication[27].
32
2. Material and Methods
Dentin and Enamel
The human tooth as one of the most important organs of human body can be detached to
2 distinct parts: the crown and the root. The part that involves in chewing food is known
as crown. The root is the other region of human tooth, which is located under the gum
line and anchors the tooth to a boney socket called alveolus. The outer surface of the root
is capped in a bone-like combination of calcium and collagen fibers called
cementum. The three major layers of each tooth are known as pulp, dentin and enamel
(Fig. 22).
Pulp: the soft combinational tissues in the central part of the tooth consists
of a vascular region called pulp. There are tiny holes in the tip of the roots
that the tiny blood vessels and nerve fibers enter to them in order to support
the hard exterior regions.
Dentin is a tough mineralized layer of tissue, which surrounds the pulp.
Since the dentin is composed of collagen fibers and hydroxylapatite (a
calcium phosphate mineral), it is much harder than the pulp. The nutrients
produced in the pulp spread through the tooth by means of dentin due to
the porous structure of it. The porous structure of dentin is the key property
of it, which is widely studied in this work. Further detailed information
will be given about dentin in subsequent sections (Fig. 23).
Enamel is the hardest substance which can be found within human body.
This nonporous material mainly made of hydroxylapatite and cap over the
dentin. The other part of this study is about this hard material.
Dentin
Dentin is a hydrated hard tissue that comprises the majority of human teeth by both weight
and volume[42]. The tissue serves as an elastic foundation for the hard, outermost enamel,
and as a protective enclosure for the central pulp. Dentin consists of microscopic
channels, called dentinal tubules, which radiate outward through the dentin from the pulp
to the exterior cementum or enamel border. By increasing the distance from the pulp, the
tubule density decreases within the dentin. A highly mineralized cylindrical cuff of apatite
mineral encircled the tubules and is regarded as
33
Figure 22 The scheme of a human tooth showing the individual layers
the peritubular dentin. Intertubular dentin employ the interstitial space between the
peritubular cuffs and is consisted of a matrix of collagen fibrils that is bound by crystalline
apatite. The collagen fibrils are dispersed in planes basically perpendicular to the
lumens[43]. The tubules contain fluid and cellular structures[44]. As a result, dentin has
a degree of permeability, which can increase the sensation of pain and the rate of tooth
decay. Dentin is traversed by a network of tubules that are oriented radially outward from
the central pulp towards the dentin–enamel junction[42]. On the other hand, type I
collagen forms a fibrous threedimensional network structure which build up the dentin
matrix. Compared to bone, the collagen matrix in dentin is more interwoven with
numerous crossing of fibrils[45]. The tubule lumens are about 1 µm in diameter and are
surrounded by a 0.5–1.5 µm hypermineralised layer of peritubular dentine (PTD) which
seems to be non-collagenous[46].
Figure 23 SEM micrograph showing dentinal tubules and porous structure of dentin
(taken from[47])
34
Enamel
Enamel is the toughest biological texture in the human body and is a composite material
comprised both a mineral and an organic phase. The mineral phase predominates (95–96
wt.%) and comprised calcium phosphate salts in the form of large hexagonal
hydroxyapatite crystals that are both carbonated and defective. Sets of similarly orientated
crystals form rod-like structures called enamel prisms, 3–6μm in cross-sectional diameter.
Prisms are separated from each other by a thin organic prism sheath and by interprismatic
enamel. The protein/organic matrix comprises approximately 1 wt.% of the enamel, and
the remaining approximately 3 wt.% is contributed by water[48].
Figure 24 SEM image of enamel surface morphology after laser and phosphoric‑acid treatment (taken from[49])
Titanium dioxide (TiO2)
The TiO2 has been commercially produced in the early twentieth century and has been
widely used as a pigment[50] in ointments, toothpastes[51], paints[52] and
sunscreens[53], [54]. Later in 1972 the photocatalytic splitting of water on a TiO2
electrode under ultra violet (UV) light has been discovered by Fujishima and Honda [66-
68]. Since then various applications have been developed for TiO2 roughly categorized in
two main “energy” and “environmental” groups moving over wide areas from
photovoltaics and photocatalysis to photo-/electrochromics and sensors[55]–[57]. These
applications based on both the properties of TiO2 alteration of TiO2 material. furthur, there
is an exponential growth of investigations on nanoscience and nanotechnology[58].
35
Various properties of materials